Imax is going to be SQRT(Pmax / R), or SQRT(50) = around 7A. Irms is something like 5A.

Based on that, do I choose an inductor that can handle 7A? Even though audio has the crest factor thingy going on (8:1) and will likely never have 7A on a continuous basis. When do the cores fail? Is it when the max current is exceeded. I believe one data sheet described the max current at the point when the inductance dropped to 80% value (core is starting to saturate).

Anybody seen the transformers from Panasonic that are meant specifically for class-D? Check out ETQA17B220. Digikey even has them for around a buck or 2.

One more thing - how does the switching frequency affect the inductance, or any other paramete? The inductors I've looked at are spec'd at 10KHz. What happens at 500KHz?

Conventional SMPS inductors are hardly useable at 500Khz. Also, conventional solid magnet wire is not very suitable for such high frequencies due to skin effect, and due to eddy currents when gapped ferrites are used.

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Originally posted by Eva Conventional SMPS inductors are hardly useable at 500Khz. Also, conventional solid magnet wire is not very suitable for such high frequencies due to skin effect, and due to eddy currents when gapped ferrites are used.

Hi Eva. I'm not sure what you're trying to tell me. Certainly many people are running class-D amps at 400Khz or so. Without being a magnetics guru, I can't say with certainty they have problems - yet they are using toroidal inductors at this power level and frequency.

Your math works out that way because Vrms for a sine wave, is Vpk / 1.414 (i.e. sqrt(2)). For peak to peak, the voltage is 2X, so you divide by 2.8185 as you wrote. But your thinking is incorrect. You don't refer to a sine wave of amplitude X as 2X, even though the peaks are at +/-X. A 1 volt sine wave is 2V peak-to-peak, average is 0V, RMS is 0.7071 [i.e. rms = 1V / sqrt(2)]

Are you confusing peak-to-peak with peak power (or peak voltage or peak current)?

Check my first post, I wrote Ppk which meant peak power - not peak-to-peak.